I just reinstalled Ubuntu 20.04 LTS and got error messages trying to play Mpeg videos. I read the instructions in "How to install the MPEG-4 AAC decoder and the H.264 decoder?" and ran a terminal for:
sudo apt-get install ubuntu-restricted-extras
The process ran as it seems it should and then ended with a EULA imposed on the terminal. "OK" was shown at the bottom of the text, but I couldn't figure out how to accept the EULA. After several tries, I, evidently foolishly, exited the terminal, ignoring the warning that a process was running and I would kill it if I exited. Of course, the decoders aren't installed/working. I tried to run it again, and then the next command:
sudo apt-get install libavcodec58 libav-tools ffmpeg
and got the following screen both times:
$ sudo apt-get install libavcodec58 libav-tools ffmpeg
[sudo] password for ij:
E: Could not get lock /var/lib/dpkg/lock-frontend. It is held by process 30816 (apt-get)
N: Be aware that removing the lock file is not a solution and may break your system.
E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), is another process using it?
I just looked at my System Monitor, but am unable to determine what process(es) are the one(s) that are causing the problem. There are two "lib" processes at the top of the list.
Am I screwed? I just spent two days reinstalling 20.04 and tweaking everything. I'm a real noobie and not comfortable with anything but clear line-by-line commands. I hope someone can help.